Three testosterone (T) implant experiments were carried out to investigate the hormonal control of seasonal hypertrophy in the sound producing, or sonic, muscles of the weakfish Cynoscion regalis. The first experiment examined the role of T in the seasonal increase in sonic muscle mass noted in adult male weakfish during the spring and summer spawning period. Specimens held over-winter were implanted with T or vehicle prior to the natural spawning season. The sonic muscle mass of the T-implanted group increased two and a half-fold over a period of 3 weeks, while that of the sham-implanted group did not change. At the end of the experiment, the cross-sectional area of the sonic muscle fibers in the T-implanted group averaged 500 microns 2, while that of the sham-implanted group averaged 300 microns 2. In a second experiment, the elevated sonic muscle mass of spawning male weakfish was sustained with pharmacological levels of plasma T for a period of 4 months following the spawning season. The sonic muscle mass of the sham-implanted group decreased to typical postseason values. In a third experiment, male and female age-0 weakfish were held over-winter and then implanted with T in advance of their first natural spawning season. The sonic muscles of the T-treated male specimens attained twice the mass of those of the sham-implanted male specimens in 3 weeks time. No sonic muscle development was noted in any of the female specimens.